Ink jet recording is a method of recording letters and figures on the surface of a recording object by ejecting ink droplets onto the object through fine nozzles. For such an ink jet recording system, for example, practically employed are a method of recording letters and symbols on the surface of a recording object by converting electronic signals into mechanical ones by the use of an electrostrictive device, followed by intermittently ejecting the ink stored in a nozzle head onto the object according to the thus-converted mechanical signals; and a method of recording letters and symbols on the surface of a recording object by rapidly heating a part of the ink stored in a nozzle head and nearest to the ejecting orifices of the nozzle to cause bubbling, and intermittently ejecting the ink onto the object by the volume expansion of the ink bubbles.
Ink for such ink jet recording is required to have various specific characteristics. For example, for printing on paper which is an recording object, the requirements of the ink to be used are that it does not cause bleeding, it quickly dries thereon, it enables uniform recording on various kinds of recording objects, and it does not mix with the neighboring ink in multi-color printing to form color prints.
For ink jet recording ink, generally used are those prepared by dissolving various kinds of water-soluble dyes in aqueous media. Recently, those prepared by dispersing pigments in aqueous media have been provided, and this is characterized in that the pigment-containing inks are superior to the water-soluble dye-containing ones in water-resistance and light-resistance.
Many conventional pigment-containing inks are to ensure the quality of printed matter by mainly suppressing their penetration into printing paper to thereby prevent the paper surface from being too much wetted with the inks and make the ink droplets stay near the paper surface. However, the inks of this type are problematic in that their applicability to all kinds of paper is low. Specifically, one problem is that the difference in the degree of bleeding depending on the kind of paper is large. In particular, for regenerated paper that comprises many unspecific paper materials, the difference in the wettability of inks with such different paper materials constituting it has an influence on the degree of ink bleeding thereon, and, as a result, the print quality is unstable. Another problem is that the inks of the type mentioned above take a long time until the printed area is dried. In particular, in multi-color printing for color prints, the neighboring inks are mixed. Still another problem is that some pigment remains on the printed paper, and therefore the printed paper is deteriorated in abrasion resistance.
To solve the problems, adding a penetrant to ink is tried for improving ink penetration into paper. For example, some examples of using glycol ethers are disclosed as follows. An example of using triethylene glycol monomethyl ether is disclosed in Japanese Patent Laid-Open No. 147861/1981; and an example of using ethylene glycol, diethylene glycol or triethylene glycol ethers is in Japanese Patent Laid-Open No. 111165/1997. Addition of diethylene glycol monobutyl ether is proposed in U.S. Pat. No. 5,156,675; addition of an acetylene glycol surfactant, Surfynol 465 (by Nisshin Chemical) is in U.S. Pat. No. 5,183,502; and a combined use of diethylene glycol monobutyl ether and Surfynol 465 is in U.S. Pat. No. 5,196,056. In U.S. Pat. No. 2,083,372, proposed is using diethylene glycol ethers in ink.
For preparing conventional pigment-containing inks, in general, pigment is dispersed in an aqueous medium by a dispersant such as surfactant or polymer dispersant. However, this is problematic in that it is not easy to improve the ink penetration while ensuring the pigment dispersion stability in ink.
In the inks of pigment dispersion prepared by dispersing pigment by a dispersant such as surfactant or polymer dispersant so as to improve the ink penetration, the pigment merely adsorbs the dispersant on its surface. Therefore, when the ink of the type is ejected through fine nozzles, the adsorbed dispersant drops off in case where it receives strong shear force, and the pigment dispersibility in the ink is lowered, thereby the ejection may tend to become unstable. In addition, when the ink is stored for a long time, the dispersibility may tend to become unstable. To solve the problems, an increase of the addition amount of the dispersant can be considered. In such a case, however, the amount of the dispersant not adsorbed by the pigment but still dissolving in the ink increases, and therefore the ink often involves the phenomena that when used in printing plain paper or regenerated paper, it readily cause bleeding and it wets the periphery of the nozzles to readily make the ejection unstable. In addition, in case where ink of a pigment dispersion prepared by dispersing a pigment by a dispersant such as surfactant or polymer dispersant to ensure improved penetration is used in printing on plain paper or regenerated paper, the pigment which is the colorant component therein tends to hardly remain on the surface of the fibers constituting the surface of the recording medium and, as a result, there is a tendency that the printed density is insufficient and the color development is not good. Moreover, in general, the dispersant not originally adsorbed by the pigment but dissolving in ink, or the dispersant having dropped off from the pigment tend to increase the viscosity of the ink, and therefore the pigment content of the ink is limited in many cases. Therefore, the ink of this type could not ensure sufficient printed density especially on plain paper or regenerated paper, and hence good color development cannot be attained and high-quality images are hardly obtained. In addition, when ink is left in heads for a long time, its dispersion stability is deteriorated and there have been cases where the ejection of the ink from the nozzles of a printer becomes difficult.
With respect to these problems, inks using pigments having subjected to some surface treatment to their surfaces have been proposed for improving ejection stability, dispersion stability, printed density and color development property. Examples of the inks using treated pigments are as follows. For example, inks using pigment particles oxidized on their surfaces are proposed in Japanese Patent Laid-Open No. 319444/1996; inks using encapsulated fine pigment particles are proposed in Japanese Patent Publication No. 94634/1995 and Japanese Patent Laid-Open No. 59715/1996; and inks using pigment particles having graft polymerized a polymer on their surfaces are proposed in Japanese Patent Laid-Open Nos. 339516/1993, 302227/1996, 302228/1996 and 81647/1996.
In addition to the above proposals, further proposed are inks using a pigment coated with a resin having film-forming property through phase conversion emulsification at room temperature, as disclosed in Japanese Patent Laid-Open Nos. 218015/1996, 295837/1996, 3376/1997, 183920/1996, 46075/1998, 292143/1998, 80633/1999, 349870/1999 and 7961/2000; and inks using a pigment coated with an anionic group-containing organic polymer compound through acid deposition, as disclosed in Japanese Patent Laid-Open Nos. 31360/1997, 217019/1997, 316353/1997, 104834/1997, 151342/1997, 140065/1998, 152424/1999, 166145/1999, 166145/1999, 199783/1999 and 209672/1999. However, inks using the colorant prepared through phase conversion emulsification or acid deposition in combination with a surfactant, as a penetrant, such as glycol ethers or acetylene glycol surfactants are still unsatisfactory especially in point of the image quality on plain paper or regenerated paper. Specifically, the resin not absorbed by the pigment exists in the ink with being dissolved therein; and the resin adsorbed by the pigment drops off to increase the amount of the resin dissolving in the ink. For these reasons, when the ink is used in printing on plain paper or regenerated paper, the pigment which is the colorant component hardly remain on the surface of paper-forming fibers on the recording medium surface. Therefore, it is difficult to obtain a sufficient printed density and the color development is not good. In addition, bleeding readily occurs. Furthermore, it tends to cause a phenomenon that the ink wets the periphery of the nozzles to readily make the ejection unstable.
The present invention is to solve at least some of the problems described above. Accordingly, an object thereof is to provide an ink jet recording ink having excellent dispersion stability and ejection stability and being capable of providing a high-quality image free from bleeding and that has high printed density and excellent color development property even on plain paper and regenerated paper.
Another object of the invention is to provide an ink jet recording ink capable of attaining satisfactory rubbing resistance not only on plain paper and regenerated paper but also on other recording media such as coated paper, etc.
A still other object of the invention is to provide an ink jet recording ink that has excellent long-term storage stability.